Generally, optical wave guides are manufactured so that optical path lengths in the process when light emitted from a light source arrives at an optical sensor part are made long and at the same time efficiency of transmittance of light in respect to optical sensor part is maximized. Optical wave guides are core configurations of optical gas sensors, and a number of publications were made public before the application of the present invention.
Korean Patent No. 10-0694635, 10-0732708, 10-1088360 and Korean Patent Laid-open Publication 2013-82482 are basically realized in an elliptical structure, and Korean Patent Laid-open Publication No. 2009-121810 and Korean Patent Laid-open Publication No. 2011-59006 comprises a condenser in front of a sensor part. Meanwhile, Korean Patent Laid-open Publication No. 2009-91433 and Korean Patent Laid-open Publication No. 2011-11307 has a reference sensor or a reference light source for improvement of reliability of sensor characteristics.
FIG. 1 is a drawing showing the characteristics of Korean Patent No. 10-0694635. First focal point and second focal point are formed in an elliptical dome shaped reflector (10). A light source (11) is positioned at the first focal point of an elliptical dome shaped reflector (10), and an optical sensor (12) is positioned at the second focal point of an elliptical dome shaped reflector (10). A flat surface reflector (13) is formed as a concaved flat mirror surface to condense infrared rays reflecting from an elliptical dome shaped reflector (10) after emitting from a light source (11). A light sensor (12) is installed horizontally on an elliptical dome shaped reflector (10) to receive all of the infrared rays reflecting from a flat surface reflector (13) and light directly irradiating from a light source (11).
Korean Patent No. 10-0694635 adopts a structure that uses only half of an elliptical dome shaped reflector (10) and directing light reflecting from the other half to an optical sensor (12) through a reflector. This structure is a structure using only less than half of light flux of an irradiating light, in the case of light irradiating and reflecting in lower flat surfaces, there are disadvantages of difficulty of adequately irradiating to an optical sensor (12) when passing though a filter attached to an optical sensor (12) due to refraction.
FIG. 2 is a drawing illustrating a main example provided by Korean Patent No. 10-1088360. According to FIG. 2, first elliptical mirror (111) and second elliptical mirror (112) of an optical wave guide (110) is formed along a portion of an entire trajectory of each first ellipse (111a) and second ellipse (112a) sharing a first focal point (111b, 112b).
A light source (120) is installed at first focal point (111b, 112b) shared by a first ellipse (111a) and a second ellipse (112a). A first light detecting window (131) and a second light detecting window (132) transmit light reflected from a first elliptical mirror (111a) and a second elliptical mirror (112a). A photo sensor part (130) detects light transmitting through a first light detecting window (131) and a second light detecting window (132). This structure has advantages of being easily manufactured in small sized structures, and is a structure able to condense without additional lens. But, the optical wave guide (110) of Korean Patent No. 10-1088360 structurally inherits disadvantages of condensing maximum of only ¼ of light from two light detecting windows.
The structure proposed in Korean Patent Laid-open Publication 2013-82482 is a structure gathering light emitted by a light source placed on a first focal point F1 of a first parabolic mirror (151) by a light detector placed on second focal point F2 of second parabolic mirror (152). According to FIG. 3, it shows advantages of measuring from two or more optical sensors are possible according to the shape and placement methods of the two parabolic mirrors (151, 152). But the structure using only two parabolic mirrors (151, 152), as proposed by J. S. Park and S. H. Yi in Sensors and Materials (thesis in year 2011), is a structure having disadvantages of not being able to effectively use light because condensing pattern shows a shape other than a round shape.
The structure proposed in Korean Patent Laid-open Publication 2009-121810, as illustrated in FIG. 4, has advantages of being able to improve optical sensing by comprising a lens (162) condensing light irradiating from a light source (161) to an optical sensor part (162). But, there are disadvantages of optical path being relatively short, and increased manufacturing costs due to installation of additional lens.
The structure proposed in Korean Patent Laid-open Publication 2011-59006, as illustrated in FIG. 5, has advantages of being able to improve light intensity by adopting a lens in front of an infrared sensor (174), but has disadvantages of increase in cost due to use of additional components and amount light arriving at an optical sensor part becoming relatively small by adopting a reflector (172) to increase optical path.
The structure proposed in Korean Patent Laid-open Publication 2011-11307, as illustrated in FIG. 6, has a reference sensor or a reference light source for improving sensor reliability. But has structural disadvantages of having lower optical sensor part (230) output compared to that of a structure with a lens because there is no special structure to condense incident infrared rays in the front-end of a photo sensor part (230).
The structure proposed in Korean Patent Laid-open Publication 2009-91433, as illustrated in FIG. 7, may periodically compensate the output of an infrared sensor (350) using a reference light source (310) and a main light source (320), in other words, multiple light sources. Also, it has a structure that may increase sensitivity of an infrared sensor (350) by making the optical path longer through multiple reflectors (361, 362, 363, 364). But, it may show disadvantages of being difficult to use for gas measurements for long wavelength ranges (>6 μm) as pattern of light arriving at an infrared sensor (350) incidents in parallel.
FIG. 8 is a drawing illustrating the relationship between blackbody radiation and light intensity. When using an infrared lamp, equation for light intensity (BT) of each wavelength range irradiating from a filament (a light source with a temperature of about 4000K) inside a vacuum glass sphere is expressed as equation (1).
                                          B            λ                    ⁡                      (            T            )                          =                                            2              ⁢                              hc                2                                                    λ              5                                ⁢                      1                                          e                                  hc                                      λ                    ⁢                                                                                  ⁢                                          k                      B                                        ⁢                    T                                                              -              1                                                          equation        ⁢                                  ⁢                  (          1          )                    
T: absolute temperature, kB: Boltzmann constant, h: Planck constant,
c: velocity of light
As expressed in FIG. 8 and equation (1), optical energy irradiating for gas detection is inversely proportional to approximately fifth power of a wavelength. When wavelength is long, output of an optical sensor part may be predicted to be marginal as intensity of incident light is small. Therefore, a structure for effectively condensing incident light to an optical sensor part is inevitable for improving sensor output.
Also, Beer-Lambert Law, which is applied broadly for infrared gas sensor manufacturing and applications, may be expressed as equation (2).I=I0·(−αxl)  equation (2)
Io: an initial light intensity, α: absorption coefficient for specific gas, x: density of gas, l: optical path.
To improve output of an infrared gas sensor, as equation (3) proposed by J. S. Park and S. H. Yi in Sensors and Materials (thesis in year 2011), it may be observed that an incident light arriving at an optical sensor part emulating a condensed shape rather than an initial optical pattern is effective.
                    V        =                                            ζ              ⁡                              (                                                      r                    i                                                        r                    d                                                  )                                      2                    ·                      (                                          -                α                            ⁢                                                          ⁢              xl                        )                                              equation        ⁢                                  ⁢                  (          3          )                    
ζ: proportional constant, ri: radius of initial optical pattern, rd: radius of optical pattern at a sensor.
Looking into items that should be considered for manufacturing optical gas sensors as expressed in formula (1), (2), (3),                1) since light intensity of a light source decreases from secular change of its own filament, it should be appropriately compensated by sensing secular change according to time,        2) and when gas with long wavelengths is to be measured, it should be a high performance sensor able to sufficiently detect light or a structure able to improve light intensity because the intensity of light irradiating from a light source is small (from equations 1 to 3),        3) since optical path should be long for sensitivity of an infrared gas sensor to generate high output voltages at identical densities, optical structures should be manufactured to have a path as long as possible, and in this instance, a state that may minimize amount absorbed when reflecting from a structure should be ensured by minimizing reflection from an optical structure,        4) and should be equipped with a characteristic of incident light arriving at an optical sensor part to be collected in the center of an optical sensor part in a radius as small as possible and should reach inside field of view of an optical sensor.        